Recent evidence suggests the possibility that enhanced inactivation of endothelium-derived nitric oxide (NO) by oxygen free radical (OFR) may cause endothelial dysfunction in heart failure (HF). To test this hypothesis, we examined the effect of antioxidant therapy on endothelium-dependent vasodilation of the coronary circulation in a canine model of tachycardia-induced HF. Endothelium-dependent vasodilation was less than that in controls, and OFR formation in coronary arterial and myocardial tissues was greater in HF dogs than those in controls. The immunohistochemical staining of 4-hydroxy-2-nonenal, OFR-induced lipid peroxides was detected in coronary microvessels of HF dogs. Intracoronary infusion of the cell-permeable OFR scavenger Tiron inhibited OFR formation and improved endothelium-dependent vasodilation in HF dogs but not in controls. The NO synthesis inhibitor N(G)-monomethyl-L-arginine (L-NMMA) diminished the beneficial effect of Tiron in HF dogs. Endothelium-independent vasodilation was similar between control and HF dogs, and no change in its response was noted by Tiron or Tiron plus L-NMMA in either group. In summary, antioxidant treatment with Tiron improved coronary vascular endothelium-dependent vasodilation by increasing NO activity in tachycardia-induced HF. Thus coronary endothelial dysfunction in HF may be, at least in part, due to increased inactivation of NO by OFR.